In the gasification of coal, oil, biomass, or other fossil fuels, or in other applications which utilize or produce other reactive gases under atmospheric or pressurized conditions, filtration of the generated synthetic gas is often required prior to use of the gas for combustion, chemical manufacturing, or other applications.
Modern gasification methods, especially, have resulted in a need for particle separators capable of efficiently and reliably separating particulate material from gases, usually high-temperature, high-pressure product gases. In fluidized bed gasification processes fine dust such as fine solid bed material, fly ash, unreacted fuel, and even reactive absorbent (i.e., for sulphur capture), is entrained with the exhausted high-temperature, high-pressure product gases from the gasification chamber. The solids have to be separated from the gases and are usually recirculated back into the gasification chamber.
In modern combined cycle power plants, where hot gases are expanded in gas turbines, efficient cleaning systems for the gases are essential for reliable operation of the gas turbines, since even very fine particulate matter in the gases cause severe erosion and fouling of the turbine blades.
The present-day concern with air pollution has, on the other hand, also created a demand for efficient dust collecting apparatus for increased capacity. Filtration can be used to collect entrained particles, aerosols, or other solids or liquids contained in the gases, to produce a cleaner product gas.
Different types of barrier filters are used to achieve the needed reliable separation of solid particulate matter from gases. In recent years gas permeable ceramic or super alloy filters have become a legitimate alternative to conventional separators (such as conventional baghouses, which are mainly used for cleaning gases of more moderate termpatures). Ceramic and super alloy filters make it possible to separate particulate matter from high-temperature, high-pressure combustible gases. These filter separators made of porous super alloys or ceramics can be built into substantially-compact and simple systems as they endure very high temperature conditions and do not need to be protected by cooling surfaces or refractory linings, and the filters can very easily be integrated into pressurized systems.
The use of barrier filters, ceramic filters, as well as conventional bag type filters, to remove particulates from dust-laden gases requires periodic cleaning of the filter. During operation a cake of dust will be collected on the surface of the filter media. Some of the dust particles clog the pores or gas passages of the filter, and the rest of them form a compact cake which rapidly builds up on the surface of the filter with resultant decrease in efficiency, or even blockage of the filtration action. As large amounts of dust are accumulated on the filter surface, a large pressure difference is built up between both sides of the filter and increased pressure, i.e., energy, is needed for passing the gas therethrough. The pressure drop across the filter media will increase, until the filter medium is regenerated, to remove the materials which have been collected on the filtration surface, so that the filtration process can be continued over a sustained operating period.
Conventional bag filters have been cleaned by shaking, agitating, vibrating or even brushing or scraping, thereby loosening dust layers accumulated on the filter medium. High-temperature, rigid ceramic filters, such as long, thin ceramic tube filters, may be very fragile and consequently would be damaged by such treatment.
One method for accomplishing the regeneration of filter elements is by periodically applying a brief or sustained reverse high pressure pulse of gas to the filter elements, in a reverse direction to the normal flow of gases in the filter. This reverse gas flow, or the shock wave which results from the momentary change in gas flow direction or volume, causes the dust cake to dislodge from the filter surface and allows removal of the dust from the system. This cleaning system is known as back-pulsing, reverse cleaning, or "soot blowing" of the filter.
The cleaning technique of choice for rigid ceramic filters is usually reverse cleaning or back-pulsing. During reverse cleaning the main separation process in the filter element section to be cleaned is stopped for a short period of time to let the cleaned gas flow back through the filter.
Compressed air, or high-pressure steam, are the standard substances used for back-pulsing. Filter tubes are cleaned by injecting compressed air pulses periodically into the tubes. The compressed air pulses release dust from the outer surface of the filter tubes. Each filter tube may have a compressed air injector of its own, or a common movable injector for several tubes has to be used in order to ensure cleaning of all filter tubes. U.S. Pat. No. 4,468,240 shows a filtering separator having a filter cleaning apparatus of the above-mentioned type. Attention is also directed to the pulse cleaning system of copending U.S. application Ser. No. 07/569,125 filed Aug. 17, 1990, the disclosure of which is hereby incorporated by reference herein.
When filtration is performed with fully oxidized or unreactive gases, air or steam can be very effective for back pulsing the filter. But, in applications such as the removal of particulates from reactive gases, such as combustible synthetic gas, derived from the gasification of biomass, coal, oil, or other substances, it is unsafe to use air or steam for back-pulsing of the filter. Air, especially at high temperature, can react uncontrollably and violently with the combustible gas and cause a fire or explosion. Steam could also react with the gas, and cause damage to certain ceramic materials used in filters.
It has been suggested to backwash filters by using gaseous filtrate, i.e., gas already cleaned in the filter, as the back-washing medium. In such processes the flow of dirty gas to the filter apparatus is controlled by the use of mechanical valves and the flow thereof is shut off immediately before the filtrate is forced back through the filter. Complex piping and valve systems are needed for forcing cleaned gas backwards through the system, and thus may be very hard to accomplish in a commercial environment. Considerable difficulty is experienced in connection with the mechanical valves that are required to frequently open and close the dirty gas feed lines and the filtrate lines. A great number of these valves are needed especially if different filter tubes or filter sections are back-pulsed separately and the operation of the filter system becomes very complex. Filtration methods utilizing a great number of valves could not be used in high-temperature operation, due to increased erosion and corrosion, especially not if the gas includes low melting alkaline or metal vapors.
It has also been suggested in U.S. Re. Pat. No. 24,954 to backwash rigid filters by introducing small charges of air and explosively combustible gas on the downstream side of the filter medium, thereby inducing a sharp explosion and setting up a shock wave for dislodging the filter cake accumulated on the filter medium. This is not a safe method for many environments, e.g. in a combi-cycle power plant.
The invention seeks to provide an improved method and apparatus for the filtration of combustible gases containing fine particles by minimizing the drawbacks of known filter surface cleaning systems, when cleaning combustible gases. The invention seeks to provide a safe method for cleaning filter medium used for filtration of combustible gases, and an efficient method for cleaning combustible gases which can easily be retrofitted to existing filters.